Operating an AC machine from a DC power source typically requires the use of a solid state inverter for establishing a rotating magnetic field in the stator of the machine. To reduce motor losses, the switching devices of the inverter are generally pulse-width-modulated (PWMed) at a relatively high frequency (such as 20 kHz) so that the current supplied to the stator windings is substantially sinusoidal. Traditionally, this is achieved with discrete digital and/or analog circuit elements, such as reference sine and triangle-wave generators and comparators.
More recently, there hall been a trend toward the use of micro-processor based controllers, in which the PWM on-times for each phase winding are stored in non-volatile memory as a function of rotor position and requested current amplitude. A co-pending U.S. patent application Ser. No. 07/975,295, filed Nov. 12, 1992, and assigned to the assignee of the present invention, is directed to such a system, based on a multi-processor micro-controller architecture such as the MC68332, manufactured by Motorola Inc., Phoenix Ariz.
A micro-processor based current control may be performed either open-loop or closed-loop. In open-loop control, the inverter switching devices are simply switched in accordance with the stored PWM on-times, as shown in the above-referenced U.S. patent application Ser. No. 07/975,295. In closed-loop control, the currents in the various phase windings are individually measured and provided as feedback for comparison with respective phase current commands. Each comparison yields an error signal, which in turn, is used to effect real-time adjustment of the PWM on-times so that the measured currents are brought into correspondence with the current commands.
Closed-loop control can be significantly more accurate than simple open-loop control, but typically requires the use of separate current sensors for the different phase windings of the machine. With certain winding configurations, one sensor may be omitted since the current in the respective phase winding may be inferred by Kirchhoff's current law.